Methods and compositions for treating subterranean zones

ABSTRACT

Methods and compositions for breaking treatment fluids utilized in the stimulation of a subterranean formation.

BACKGROUND

The present embodiment relates to methods and compositions for treatingsubterranean zones in formations penetrated by well bores utilizingstrongly delayed polymer breakers.

Treating fluids containing polymer breakers are used in a variety ofoperations and treatments in oil and gas wells. An example of a wellcompletion treatment which utilizes a polymer breaker in a highviscosity fluid is known in the art as gravel packing. In gravel packingtreatments, solid gravel particles such as sand are carried by way ofthe well bore to a subterranean zone in which a gravel pack is to beplaced by a viscous gelled carrier fluid. That is, particulate solids(referred to in the art as gravel) are suspended in the high viscositycarrier fluid at the surface and carried to the subterranean zone inwhich the gravel pack is to be placed. Once the gravel is placed in thezone, the viscous carrier fluid is broken (the viscosity is reduced) andrecovered (returned to the surface) by including a delayed polymerbreaker, i.e., a viscosity reducing agent, in the carrier fluid. Thegravel pack produced functions as a filter to separate formation solidsfrom produced fluids while permitting the produced fluids to flow intoand through the well bore.

In open hole gravel packing procedures, a non-viscous carrier fluid canbe used that includes a polymer breaker which breaks down drill-in fluidfilter cake left on the walls of the open hole well bore from the wellbore drilling operation. The carrier fluid for open hole gravel packingcan also be viscosified. In that case, the delayed breaker in thecarrier fluid breaks the carrier fluid and the filter cake so that thecarrier fluid and the filter cake can be removed from the subterraneanzone.

The well completion procedures utilizing polymer breakers can beimproved if the polymer breakers have a delayed reaction on theviscosity of the treatment fluid or on the degradation of the filtercake. For example, breaker compositions that include sodium persulfateand lithium hypochlorite which generally-provide delayed breaks in therange of 0 to 2 hours are utilized in these operations. Recently,however, it has been recognized that even greater improvements to andsimplification of well completion procedures can be realized if thebreaks in viscosity of a carrier fluid or filter cake integrity can beeven more strongly delayed. In this context and as used herein, the term“strongly delayed” as used in connection with a break in viscosity of acarrier fluid or filter cake integrity means a break delay of more than3 hours.

In well temperatures above 150° F., t-butyl hydroperoxide has been foundto function as a strongly delayed breaker. However, in well temperaturesbelow 150° F., it has proven to be difficult to obtain strongly delayed,controllable break times of biopolymer components such as xanthan andsuccinoglycan gums of viscosified fluids or filter cakes. Attempts toobtain strongly delayed, controllable break times by reducing theconcentration of the breaker generally results in incomplete breaks ofthe polymer and may be damaging to the permeability of the producingzone.

Thus, there is a need for treating fluid breaker systems which canprovide controllable, strongly delayed breaks of biopolymer viscosifiedaqueous well treating fluids and filter cakes at temperatures rangingfrom 80 to 150° F.

DETAILED DESCRIPTION

The methods and compositions of the present embodiment provide a meansfor treating subterranean zones using water based treating fluids whichcontain strongly delayed water soluble polymer breakers. According toone method of the present embodiment, a water based viscous treatingfluid composition is provided comprising water, a viscosity increasingpolymer and a strongly delayed polymer breaker composition thatcomprises a mixture of a hydrogen peroxide source, a ferrous ion sourceand a chelating agent. The viscous treating fluid composition isintroduced into a subterranean zone by way of a well bore penetratingthe zone and the strongly delayed polymer breaker is allowed to breakthe viscous treating fluid into a thin fluid of reduced viscosity.Thereafter, the treating fluid is recovered from the subterranean zone.

According to another method of the present embodiment, a water basednon-viscous treating fluid composition is provided comprising water anda strongly delayed polymer breaker composition that comprises a mixtureof a hydrogen peroxide source, a ferrous ion source and a chelatingagent. The treating fluid composition is introduced into a subterraneanzone by way of an open hole well bore penetrating the zone that hasfilter cake on the walls thereof. The delayed polymer breaker in thetreating fluid composition is then allowed to break the filter cake.Thereafter, the treating fluid and broken filter cake are removed fromthe subterranean zone.

According to yet another method of the present embodiment, a water basedviscous treating fluid composition is provided comprising water, aviscosity increasing polymer and a strongly delayed polymer breaker thatcomprises a mixture of a hydrogen peroxide source, a ferrous ion sourceand a chelating agent. The viscous treating fluid composition isintroduced into a subterranean zone by way of an open-hole well borepenetrating the zone that has filter cake on the walls thereof. Thestrongly delayed polymer breaker in the viscous treating fluid is thenallowed to break the viscous treating fluid and the filter cake.Thereafter, the broken treating fluid and broken filter cake are removedfrom the subterranean zone.

A water based treating fluid composition of the present embodimentcomprises water and a strongly delayed polymer breaker comprising amixture of a hydrogen peroxide source, a ferrous ion source and achelating agent. According to a preferred embodiment, the treating fluidcomposition includes a viscosity increasing polymer.

The water based treating fluid compositions of the present embodimentcan be utilized for forming gravel packs in a subterranean zone or forcarrying out other completion, stimulation or work over procedures. Ingravel packing or other applications in subterranean zones, water basedtreating fluid compositions are often utilized. In some of theapplications, the treating fluid compositions must have high viscositieswhich are provided by viscosity increasing polymers. In order to recoversuch viscous treating fluid compositions from the subterranean zones,strongly delayed polymer breakers are included in the fluids. In otherapplications, the water based treating fluid compositions are placedinto well bores with drill-in fluid filter cake on the well bore wallsand at least one function of the water based treating fluid containing astrongly delayed polymer breaker is to degrade the filter cake.

A strongly delayed breaker system having a desired level ofcontrollability is accomplished by preparing a composition whichincludes a hydrogen peroxide source, a ferrous ion source and achelating agent capable of chelating iron. This multi-component breakercomposition provides additional parameters for control by changing theconcentration and relative ratios of the different components.

Generally, to break a polymer such as xanthan gum requires thegeneration of a certain number of cleavages in the polymer backbone orpolymer branches so as to break the polymer and cause the desiredreduction in viscosity. Accordingly, the concentration of oxidizerneeded to break the xanthan gum in a xanthan gum solution can bedetermined based on the amount of xanthan gum in the solution. Statedanother way, the number of oxidizer molecules is determined by thenumber of cleavages in the polymer backbone needed to achieve a desiredreduction in viscosity.

To obtain a strongly delayed break of the polymer or a slow rate ofbreak down of the polymer, with a strong oxidizer, would require areduction of the concentration of the oxidizer. However, there is alimit to the degree to which the concentration of the oxidizer can bereduced because as noted above, there are a certain number of cleavagesin the polymer backbone that are necessary to achieve the desiredreduction in viscosity. Therefore, to achieve a strongly delayed polymerbreaker system, a control mechanism other than the concentration ofoxidizer alone is necessary. Such control is provided by the activatorsystem of the compositions of the present embodiment. According to thestrongly delayed polymer breaker compositions of the present embodiment,strongly delayed polymer breaks are accomplished, not by reducing theoxidizer alone, but by changes to the concentrations of the componentsof the activator.

It will be understood that if a strong oxidizer is added to a viscousgel, it would fairly quickly reduce the viscosity and would not resultin a strongly delayed break of the gel. Instead according to thestrongly delayed polymer breaker compositions of the present embodiment,a weak oxidizer (hydrogen peroxide at the temperatures underconsideration) is used that is not capable of breaking the xanthanpolymer on its own and the hydrogen peroxide is slowly catalyzed to astrong oxidizer. According to the strongly delayed polymer breakercompositions of the present embodiment, the ferrous ion source incombination with hydrogen peroxide generates a hydroxyl radical that isa strong oxidizer. The ferrous ion is not consumed in this reaction (thehydrogen peroxide is) so the concentration of the ferrous ion drives therate at which hydroxyl radicals are generated. In such a system,however, at neutral pH, ferric iron will precipitate from solution asiron hydroxide. So, to prevent such precipitation, citrate anion isadded to complex with the ferrous ion. However, citrate tends tointerfere with the generation of the hydroxyl radical. Therefore, themore citrate that is added, the more interference is caused and theslower the generation of the stronger oxidizer. Consequently, theferrous ion and the citrate anion constitute the activator system forthe strongly delayed polymer breaker compositions of the presentembodiment.

The water utilized in the well treating fluids of this embodiment can befresh water or salt water. The term “salt water” is used herein to meanunsaturated salt solutions and saturated salt solutions including brinesand seawater. Generally, salt is added to the water to provide claystability and to increase the density of the water based fluid. Examplesof salts that can be used include, but are not limited to, sodiumchloride, sodium bromide, calcium chloride, potassium chloride, ammoniumchloride and mixtures thereof. The salt or salts used can be present inthe salt water in a concentration up to about 66% by weight thereof andthe salt water can have a density up to about 15.5 pounds per gallon.The water may include any of the other conventional additives such asproppants, pH control agents, bactericides, clay stabilizers,surfactants and the like which do not adversely react with the othercomponents of the viscosified aqueous well treating fluids to inhibitperformance of the desired treatment upon a subterranean formation.

When a viscous treating fluid composition is utilized in accordance withthis embodiment, various viscosity increasing polymers can be includedin the treating fluid composition. A preferred group of viscosityincreasing polymers include biopolymers such as xanthan andsuccinoglycan gums.

Preferably, such biopolymers are generally present in the viscous fluidcompositions in an amount in the range of from about 0.25% to about 1.5%by weight of the water in the compositions.

A preferred strongly delayed polymer breaker composition according tothe present embodiment is effective in breaking viscosified aqueous welltreating fluids at ambient temperature and at a pH greater than 3.0 to4.0, preferably at a pH of 7.0. The composition of the strongly delayedpolymer breaker includes a mixture of ferrous ions and hydrogen peroxidewhich promotes the oxidation of organic compounds by the generation ofthe hydroxyl radical from the hydrogen peroxide. The production of thehydroxyl radical, which is a strong oxidizing agent, is catalyzed by thepresence of the ferrous ions.

The source of ferrous ions in the preferred strongly delayed polymerbreaker composition, may be one or more ferrous compounds such as iron(II) sulfate heptahydrate (FeSO₄.7H₂O), iron (II) chloride (FeCl₂), andiron (II) gluconate. However, it will be understood that other sourcesof ferrous ions may also be used.

Also, the source of hydrogen peroxide in the preferred strongly delayedpolymer breaker composition, is sodium perborate tetrahydrate(NaBO₃.4H₂O) or a solution of concentrated hydrogen peroxide (H₂O₂).

Preferably, the strongly delayed polymer breaker composition alsoincludes sodium chloride (NaCl) which increases the rate of oxidation oforganic compounds by the hydroxyl radical.

Generally, when compositions which include a source of ferrous ions anda source of hydrogen peroxide reach a pH of greater than 3.0 to 4.0,ferric ions in equilibrium with ferrous ions precipitate with hydroxylions as ferric hydroxide. The precipitation of ferric hydroxide from thestrongly delayed polymer breaker composition of the present embodimentis undesirable and preferably is avoided. The precipitation of such ironcompounds from solution, preferably is prevented or retarded by theaddition to the strongly delayed polymer breaker composition of achelating agent which forms a complex with the ferric ions thuspreventing or retarding such precipitation. The chelating agentpreferably keeps the ferric ions in solution without overly interferingwith the Fe²⁺/Fe³⁺ redox activity during hydroxyl radical formation.Preferred chelating agents which keep the ferric ions in solution and donot overly interfere with the redox activity include but are not limitedto citric acid, sodium citrate and iminodiacetic acid. Preferably, thestrongly delayed polymer breaker composition includes a molar excess ofthe chelating agent relative to the ferrous ions so as to avoid theprecipitation of ferric hydroxide and the ratio may also be used as oneof the mechanisms to control the rate of polymer degradation. Mostpreferably, the strongly delayed polymer breaker composition includes aratio of chelating agent to ferrous ions of from 3:1 to 6:1.

The ratio of chelating agent to ferrous ions in the strongly delayedpolymer breaker composition of the present embodiment may be utilized tovary the break time of the viscosified aqueous well treating fluids. Inaddition, by varying the concentrations of the source of hydrogenperoxide, the chelating agent and ferrous ions, a high degree offlexibility and control over the break time of the viscosified aqueouswell treating fluids can be maintained.

In the practice of the present embodiment, the strongly delayed polymerbreaker composition can be injected with a gravel pack fluid or, ifadded to a carrier fluid, injected into a subterranean formation priorto, simultaneously with, or subsequent to injection of the gravel packfluid. Generally, the strongly delayed polymer breaker composition willbe admixed with a carrier fluid.

The amount of strongly delayed polymer breaker composition used is thatamount required to reduce the viscosity of the viscosified aqueous welltreating fluids at a static temperature in the range of from about 80°F. to about 150° F. to a preselected lower viscosity or to a completebreak. The optimum or effective amount of the strongly delayed polymerbreaker composition employed in accordance with the present embodimentdepends on factors such as the particular gelling agent and itsconcentration, the formation temperature and other factors. Typically,however, the strongly delayed polymer breaker composition is employed inthe range of from about 0.01 to about 500 pounds per 1000 gallons ofviscosified aqueous well treating fluids.

Suspended particulate solids such as gravel for forming gravel packs canbe included in the water based viscous treating fluid compositions. Thegravel particles are suspended in a viscous treating fluid compositionand are deposited in a subterranean zone when the viscosity of theviscous treating fluid composition is broken. Examples of usefulparticulate solids include, but are not limited to, graded sand,bauxite, ceramic materials, glass materials, walnut hulls, polymer beadsand the like. Of the various particulate solids that can be used, gradedsand is generally preferred.

A preferred method of this invention for treating a subterranean zonecomprises the steps of: (a) providing a water-based, viscous treatingfluid composition comprising water, a viscosity increasing polymer and awater-soluble, strongly delayed polymer breaker composition thatcomprises a hydrogen peroxide source, a ferrous ion source and achelating agent; (b) introducing the viscous treating fluid compositioninto the subterranean zone through a well bore penetrating thesubterranean zone; and (c) allowing the strongly delayed polymer breakercomposition to break the viscous treating fluid composition into a thinfluid of reduced viscosity so that it can be removed from thesubterranean zone.

Another preferred method of treating a subterranean zone comprises thesteps of: (a) providing a water-based, non-viscous treating fluidcomposition comprising water and a water-soluble, strongly delayedpolymer breaker composition that comprises a hydrogen peroxide source, aferrous iron compound and a chelating agent; (b) introducing thenon-viscous treating fluid composition into the subterranean zone by wayof an open hole well bore penetrating the subterranean zone, the wellbore having filter cake on the walls thereof; and (c) allowing thestrongly delayed polymer breaker composition to break the filter cake sothat the treating fluid and the broken filter cake can be removed fromthe subterranean zone.

Yet another preferred method of treating a subterranean zone comprisesthe steps of: (a) providing a water based viscous treating fluidcomposition comprising water, a viscosity increasing polymer and a watersoluble strongly delayed polymer breaker composition that comprises amixture of a hydrogen peroxide source, a ferrous iron compound and achelating agent; (b) introducing the viscous treating fluid compositioninto the subterranean zone by way of an open hole well bore penetratingthe subterranean zone that has filter cake on the walls thereof; and (c)allowing the strongly delayed polymer breaker composition in the viscoustreating fluid to break the viscous treating fluid and the filter cakeso that the broken treating fluid and the broken filter cake can beremoved from the subterranean zone.

A preferred water-based treating fluid composition of this embodimentcomprises: water and a strongly delayed polymer breaker compositioncomprising a mixture of a hydrogen peroxide source, a ferrous ironcompound and a chelating agent.

Another preferred water-based treating fluid composition comprises:water, a viscosity increasing polymer and a strongly delayed polymerbreaker composition comprising a mixture of a hydrogen peroxide source,a ferrous iron compound and a chelating agent.

In order to further illustrate the methods of this invention, thefollowing examples are given.

EXAMPLE 1

Solutions of xanthan gum in water were prepared by heating to 85° C. toensure polymer hydration and then cooling to room temperature. Knownamounts of such xanthan solutions were then mixed with a breakercomposition that included various quantities of Fe²⁺/citrate solutionsand hydrogen peroxide as indicated below in Table 1. The solutions allcontained 0.05% xanthan gum and had a pH of between 4 and 5. Thehydrogen peroxide was diluted from a 3% solution.

TABLE 1 Ferrous ion conc. Citrate conc. Hydrogen peroxide Solution Mol/lMol/l % w/v A 5.2 × 10⁻⁴   1 × 10⁻³ 0.075 B 1.7 × 10⁻⁴ 3.2 × 10⁻⁴ 0.075C 1.7 × 10⁻⁴ 3.2 × 10⁻⁴ 0.225 D 1.7 × 10⁻⁴ 3.2 × 10⁻⁴ 0.75 E 5.2 × 10⁻⁴  1 × 10⁻³ 0.75

The mixtures of the xanthan gum solutions and breaker compositions werethen placed in a standard Ubbelohde dilution viscometer. The viscosityof the mixtures was dominated by the xanthan gum. Flow times in excessof that for the solvent (water) are a direct measure of the molecularmass of the xanthan gum, such that decreasing flow times are a measureof the degradation of the xanthan gum. The flow times shown in Table 2were measured within 5 minutes of mixing and until they approached thatof the solvent. The solvent (water) flow time was 20.5 seconds. The datashown in Table 2, measured at ambient temperature, i.e. about 72° F.,clearly show that xanthan gum degrades at ambient temperature in thepresence of the breaker composition which included Fe²⁺/citratesolutions and hydrogen peroxide.

TABLE 2 Solution A Solution B Solution C Solution D Solution E Time ofTime of Time of Time of Time of reaction Flow time reaction Flow timereaction Flow time reaction Flow time reaction Flow time (min) (s) (min)(s) (min) (s) (min) (s) (min) (s) 10 60 10 53.5 10 55 10 51.9 10 60 4048.3 30 49.6 30 51.4 30 48.5 20 51.2 80 36.7 60 47 60 48.7 60 43 30 42100 32 90 45 90 45 90 36.4 40 34.2 110 43.5 120 42 120 31.3 60 27.7 16028.1 70 26.2

At the concentrations used, the rate of degradation of xanthan gumappears to be a function of the relative concentrations of Fe²⁺/citrate.

EXAMPLE 2

The following standard xanthan gum based mud was prepared for thepurpose of evaluating the effectiveness of various breaker compositionsset forth in Table 3 below:

TABLE 3 Component Amount Tap water  332 mL Potassium chloride (KCl) 10.5g. Xanthan Gum 0.85 g. Starch  7.4 g. CaCO₃ with 5 micron mediandiameter   10 g. CaCO₃ with 25 micron median diameter   25 g. Magnesiumoxide buffer 0.18 g.

The muds were prepared by adding the tap water to the mix cup of aHamilton Beach mixer and placed on the mixer set at high shear rate(18,000 rpm). Next the KCl was added to the tap water. The xanthan andstarch were then slowly added and left to mix at high speed for 15minutes. The other components (calcium carbonate and magnesium oxide)were then added in the order set forth above. The mud was then placed ina sealed jar and placed in a roller oven (Fann Model 701) at 150° F. for16 hours. After such hot rolling the muds were then ready for use as atest substrate as noted below.

Various breaker compositions were prepared according to the formulationsset forth in Table 4:

TABLE 4 Component Amount Amount Amount Tap water   330 mL  500 mL  500mL Sodium perborate tetrahydrate  3.96 g. 6.00 g. 6.00 g. Citric acid 1.23 g. 1.86 g. 0.94 g. Sodium chloride (NaCl)  0.6 g. 1.00 g. 1.00 g.Iron sulfate heptahydrate  0.59 g. 0.45 g. 0.45 g. Citric acid:ironsulfate  3:1  6:1  3:1 (molar ratio) Break time 5–5.5 hours   5 hours 4.5 hours

The sodium perborate tetrahydrate, citric acid, sodium chloride and ironsulfate heptahydrate are commercially available from Sigma-Aldrich, Inc.

The breaker compositions were prepared by adding the tap water to asuitable beaker, a magnetic stir bar was placed in the beaker and thebeaker was placed on a stir plate. The sodium perborate tetrahydrate,citric acid and sodium chloride were added to the beaker in the orderset forth above. A pH probe was inserted into the solution once allcomponents were dissolved. The pH of the composition was adjusted byslowly adding a 25% sodium hydroxide solution to raise the pH of thecomposition to 7. Then the iron sulfate heptahydrate was added tocomplete the preparation of a breaker composition. The molar ratios ofcitric acid to ferrous ion in the breaker composition of these exampleswere 3 to 1 and 6 to 1 as given in Table 4.

Approximately 300 mL of the drilling fluid was placed in a Fann Model90B dynamic filtration system operating at 150° F. at 800 psi and 500psi differential pressure across a 35 micron filter core. The Fann Model90B is a dynamic filtration system for testing the filter cake buildingproperties of drilling fluids. The machine was then programmed to applythe system pressure, differential pressure, temperature and shear rateneeded to build a filter cake. The mud was then removed from the machineand the filter cake was left in place against the core material. Thebreaker solution was then placed in the machine and the machine wasprogrammed to monitor the leak off rate through the filter cake at 150°F., 800 psi system pressure and 50 psi differential pressure. Anincrease in leak off rate was an indication of the degradation of thefilter cake. The point at which an increased leak off rate was recordedis given in Table 4 as “Break Time.” A detailed description of the useof a Fann Model 90B for such testing is described in SPE (Society ofProfessional Engineers) Paper No. 68968 entitled “Laboratory Device forTesting of Delayed-Breaker Solutions on Horizontal Wellbore FilterCakes” the entire disclosure of which is hereby incorporated herein.

Although only a few exemplary embodiments have been described in detailabove, those skilled in the art will readily appreciate that many othermodifications are possible in the exemplary embodiments withoutmaterially departing from the novel teachings and advantages describedherein. Accordingly, all such modifications are intended to be includedwithin the scope of the following claims.

1. A method of treating a subterranean zone, comprising: providing awater-based, viscous treating fluid composition comprising: water, aviscosity increasing polymer, and a water-soluble strongly delayedpolymer breaker composition, the water-soluble strongly delayed polymerbreaker composition comprising: a hydrogen peroxide source, a ferrousion source, and a chelating agent; wherein the water-soluble, stronglydelayed polymer breaker composition comprises a molar excess of thechelating agent relative to the ferrous ion source; introducing theviscous treating fluid composition into the subterranean zone through awell bore, penetrating the subterranean zone, and allowing the stronglydelayed polymer breaker composition to break the viscous treating fluidcomposition into a thin fluid of reduced viscosity.
 2. The method oftreating a subterranean zone according to claim 1 wherein the hydrogenperoxide source is selected from the group consisting of sodiumperborate tetrahydrate and hydrogen peroxide.
 3. The method of treatinga subterranean zone according to claim 1 wherein the ferrous ion sourceis selected from the group consisting of iron (II) sulfate heptahydrate,iron (II) chloride and iron (II) gluconate.
 4. The method of treating asubterranean zone according to claim 1 wherein the chelating agent isselected from the group consisting of citric acid, sodium citrate andiminodiacetic acid.
 5. The method of treating a subterranean zoneaccording to claim 1 wherein the water-soluble, strongly delayed polymerbreaker composition comprises a molar ratio of the chelating agent tothe ferrous ion source of from 3:1 to 6:1.
 6. The method of treating asubterranean zone according to claim 1 wherein the water-soluble,strongly delayed polymer breaker composition further comprises sodiumchloride.
 7. The method of treating a subterranean zone according toclaim 1 wherein the water-soluble, strongly delayed polymer breakercomposition has a pH in the range of from about 3 to about
 7. 8. Themethod of treating a subterranean zone according to claim 1 wherein theviscosity increasing polymer comprises a polysaccharide.
 9. The methodof treating a subterranean zone according to claim 8 wherein theviscosity increasing polymer comprises a polysaccharide selected fromthe group consisting of biopolymers and modified gums or celluloses andderivatives thereof.
 10. The method of treating a subterranean zoneaccording to claim 9 wherein the viscosity increasing polymer comprisesxanthan gum.
 11. The method of treating a subterranean zone according toclaim 1 wherein the temperature of the subterranean zone ranges fromabout 80° F. to about 150° F.
 12. A method of treating a subterraneanzone, comprising: providing a water-based, non-viscous treating fluidcomposition comprising: water, and a water-soluble strongly delayedpolymer breaker composition, the water-soluble strongly delayed polymerbreaker composition comprising: a hydrogen peroxide source, a ferrousion source, and a chelating agent; wherein the water-soluble, stronglydelayed polymer breaker composition comprises a molar excess of thechelating agent relative to the ferrous ion source; introducing thenon-viscous treating fluid composition by way of an open hole well borepenetrating the subterranean zone, wherein the well bore has filter cakeon the walls thereof, and allowing the strongly delayed polymer breakercomposition to break the filter cake whereby the treating fluid and thebroken filter cake can be removed from the subterranean zone.
 13. Themethod of treating a subterranean zone according to claim 12 wherein thehydrogen peroxide source is selected from the group consisting of sodiumperborate tetrahydrate and hydrogen peroxide.
 14. The method of treatinga subterranean zone according to claim 12 wherein the ferrous ion sourceis selected from the group consisting of iron (II) sulfate heptahydrate,iron (II) chloride and iron (II) gluconate.
 15. The method of treating asubterranean zone according to claim 12 wherein the chelating agent isselected from the group consisting of citric acid, sodium citrate andiminodiacetic acid.
 16. The method of treating a subterranean zoneaccording to claim 12 wherein the water-soluble, strongly delayedpolymer breaker composition comprises a molar ratio of the chelatingagent to the ferrous ion source of from 3:1 to 6:1.
 17. The method oftreating a subterranean zone according to claim 12 wherein thewater-soluble, strongly delayed polymer breaker composition furthercomprises sodium chloride.
 18. The method of treating a subterraneanzone according to claim 12 wherein the water-soluble, strongly delayedpolymer breaker composition has a pH in the range of from about 3 toabout
 7. 19. The method of treating a subterranean zone according toclaim 12 wherein the temperature of the subterranean zone ranges fromabout 80° F. to about 150° F.
 20. A method of treating a subterraneanzone, comprising: providing a water-based, viscous treating fluidcomposition comprising: water, a viscosity increasing polymer, and awater-soluble strongly delayed polymer breaker composition, thewater-soluble strongly delayed polymer breaker composition comprising: ahydrogen peroxide source, a ferrous ion source, and a chelating agent;wherein the water-soluble, strongly delayed polymer breaker compositioncomprises a molar excess of the chelating agent relative to the ferrousion source; introducing the viscous treating fluid composition into thesubterranean zone by way of an open hole well bore penetrating thesubterranean zone, wherein the well bore has filter cake on the wallsthereof, and allowing the strongly delayed polymer breaker compositionin the viscous treating fluid to break the viscous treating fluid andthe filter cake whereby the broken treating fluid and the broken filtercake can be removed from the subterranean zone.
 21. The method oftreating a subterranean zone according to claim 20 wherein the hydrogenperoxide source is selected from the group consisting of sodiumperborate tetrahydrate and hydrogen peroxide.
 22. The method of treatinga subterranean zone according to claim 20 wherein the ferrous ion sourceis selected from the group consisting of iron (II) sulfate heptahydrate,iron (II) chloride and iron (II) gluconate.
 23. The method of treating asubterranean zone according to claim 20 wherein the chelating agent isselected from the group consisting of citric acid, sodium citrate andiminodiacetic acid.
 24. The method of treating a subterranean zoneaccording to claim 20 wherein the water-soluble, strongly delayedpolymer breaker composition comprises a molar ratio of the chelatingagent to the ferrous ion source of from 3:1 to 6:1.
 25. The method oftreating a subterranean zone according to claim 20 wherein thewater-soluble, strongly delayed polymer breaker composition furthercomprises sodium chloride.
 26. The method of treating a subterraneanzone according to claim 20 wherein the water-soluble, strongly delayedpolymer breaker composition has a pH in the range of from about 3 toabout
 7. 27. The method of treating a subterranean zone according toclaim 20 wherein the viscosity increasing polymer comprises apolysaccharide.
 28. The method of treating a subterranean zone accordingto claim 27 wherein the viscosity increasing polymer comprises apolysaccharide selected from the group consisting of biopolymers andmodified gums or celluloses and derivatives thereof.
 29. The method oftreating a subterranean zone according to claim 28 wherein the viscosityincreasing polymer comprises xanthan gum.
 30. The method of treating asubterranean zone according to claim 20 wherein the temperature of thesubterranean zone ranges from about 80° F. to about 150° F.